Aerosols have a variety of uses in modern society. Consumers typically use aerosols in convenient size aerosol dispensers. There are also industrial applications that use larger aerosol dispensers, on the order of five gallons, that use hose and gun configurations to dispense the aerosols. In particular, aerosol adhesives are used in large dispensers in order to apply adhesive to bond various surfaces. Adhesives can come in various forms such as natural, organic, vegetable and synthetic. There are several classes of polymeric based synthetic adhesives such as thermosetting adhesives, thermoplastic resins and elastomeric adhesives to name a few. Typically aerosols and aerosol adhesives emit volatile organic compounds (VOCs) during their use.
In many localities, VOCs are regulated. VOC regulations govern both the solvent which the polymer is dissolved and the propellant. Most organic solvents are VOCs. Since aerosol adhesives emit VOCs in their use, aerosol adhesives can fall within these regulations. Regulations can differ depending on the locality, the compounds used and the size of the dispensing container. The compounds typically of concern are the propellants that are primarily contain hydrocarbons and dimethyl ether. The hydrocarbons typically include propane, butane and isobutane, which are all typically flammable. These propellant gases are widely used in aerosols. These hydrocarbons also typically act as a solvent when compressed because they become liquids. They are referred to as liquified gases. Other propellants, which are also liquified gases include chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). A variety of other compressible gases such as carbon dioxide, nitrous oxide, nitrogen and air may be used as propellants. These gases are referred to as compressible gases because at normal working pressures for aerosol formulators they do not form liquids.
As mentioned, the regulations can differ. For example, in California, there are two levels for regulating VOC emissions. One level is the State of California, and the other is the local air districts. Many of the local air districts do not classify aerosol containers as aerosol if they weigh more than one pound, rather, they classify them as bulk or liquid contact adhesives. The VOC regulations for bulk or liquid contact adhesives are typically much more stringent than the VOC regulations for aerosols. Therefore, many manufacturers have gone to manufacturing water based products because they can not manufacture compliant adhesive formulation.
Finally, a typical formula for aerosol adhesives is:
28 lb. of adhesive (30 percent solids—12 percent SIS rubber and 18 percent resin)
12 lb. of hydrocarbon propellant (50 percent propane and 50 percent isobutane)
The solvent system typically used to dissolve the rubber and resin can include methylene chloride. In this formulation, typically, the adhesive concentrate is prepared by the 28 lbs of the rubber resin mixture being dissolved in a solvent, which is then pumped into a large aerosol cylinder. In addition, 12 pounds of propellant (a blend of propane and isobutane) is typically added. This adhesive has the desired characteristics but, however, it is high in VOC due to the propellent. Furthermore, it has toxicity problems due to the large amount of chlorinated solvent also typically included. Typically, in aerosol adhesives the solvent chosen from dissolving the adhesive is a chlorinated solvent. The solvent must dissolve all of the components of the adhesive. One necessary ability of the solvent is the ability to dissolve the polymer and the resin. Another necessary property of the solvent is that it should evaporate quickly. These properties allow the end user to adhere the surfaces as quickly as possible. Typically, the chlorinated solvents are methylene chloride and bromo-propane. Although these compounds can be used, there are certain health hazards associated with them.
Furthermore, in general, can-based systems cannot maintain high pressures. Can-based systems typically maintain 50 psi. In additions, typical regulations require that cans maintain a pressure of 200 psi at a temperature of 130° F. Therefore, high solids and high viscosity formulations and blends typically cannot be effectively sprayed from cans having such low pressures.